INTRODUCTION. XI 



The capitate tentacle, of which we have good examples 

 in Coryne and Clavatella, bears on its summit a globular 

 head, consisting of a collection of thread-cells a formi- 

 dable battery of offensive weapons, which is brought to bear 

 on any passing prey. The arm is also endowed with vigo- 

 rous percussive power, and when its numerous poisoned 

 threads are brought into play, it can hardly fail to arrest 

 and paralyze any of the smaller creatures that may come 

 within its range. (Plate VII. fig. 1 6.) 



The thread-cells, which bear so important a part in the 

 Hydroid economy, exhibit many modifications. They 

 occur in the ectodermal layer, and are present in as- 

 tonishing profusion, not only on the tentacles, but in other 

 portions of the structure. They consist of minute sacs 

 imbedded in the flesh and filled with fluid, which contain 

 a long and delicate thread, capable of being projected with 

 considerable force and inconceivable rapidity. These 

 threads bury themselves in any soft substance against 

 which they may be directed, and, it is supposed, convey 

 into the wound which they make some poisonous fluid. 



The thread-cell is a most interesting piece of structure. 

 The long dart which it encloses is borne on a continuation 

 of the inner wall of the sac (the " sheath " of some writers, 

 the " axial body" of others), which is often covered with 

 barbs. (Woodcut, fig. iii. a.) When retracted, the thread is 

 spirally coiled within the cell and sometimes wound round 

 the sheath. (Woodcut, fig. iii. c.) Two kinds of thread- 

 cell are often met with on the same species. Besides the 

 formidable instruments with which the tentacles are armed, 

 large bean-shaped cells are sometimes crowded together in 

 immense quantities, as, for example, in the ectoderm of 

 the coenosarc in Hydranthea, and in the outer covering of 



